In boiling water nuclear reactors, fuel rods are grouped together in an essentially open-ended tubular flow channel, typically referred to as a fuel assembly or bundle. A plurality of fuel assemblies are positioned in the reactor core in a matrix and a coolant/moderator flows upwardly about the fuel rods for generating steam. The fuel rods are supported in each assembly between upper and lower tie plates. One or more water rods are usually located interiorly within each fuel rod bundle. Spacers are employed at predetermined elevations along each fuel bundle to restrain the fuel rods from bowing or vibrating during reactor operation.
Typical spacers often include a plurality of ferrules arranged in side-by-side relation and secured, for example, by welding to one another to form a support matrix for the fuel rods. Generally, the role of the spacer in a fuel bundle assembly is to maintain the rods in fixed lateral locations relative to one another. These spacers are secured or captured in a manner preventing their movement relative to the longitudinal axis of the bundle during operation of the reactor. Conventional methods of maintaining the fuel rod spacers in position along the axis of the bundle include the use of a pair of tabs welded to the water rods engaging a spacer along its top and bottom, preventing axial movement of the spacer relative to the water rod in either axial direction. These spacers are assembled onto the water rod by relatively rotating the spacers and water rod such that the tabs pass through an opening in the spacer structure. The spacer is then rotated to an orientation such that part of the spacer structure is axially contacting one of the two tabs, the lower tab being preferred. In the assembled bundle, the water rod is typically prevented from rotation by a square lower end plug received in a square hole in the lower tie plate.
There are, however, occasions where the fuel bundle is unchanneled. By unchanneled is meant that the fuel bundle assembly does not have the surrounding channel. For example, the bundles are unchanneled during initial fabrication when the spacers are secured to the water rod. Unchanneled bundles are relatively flexible with respect to torsion about the bundle axis. Thus, twist can occur in unchanneled bundles from normal handling of the bundles. If the twist is large enough, certain of the spacers, e.g., the upper spacers can be rotated to such an extent that the water rod tabs locate over and under the openings in the spacer structure used during assembly and no longer provide axial restraint. Also, for irradiated bundles where the friction forces between the spacer springs and the rod is significantly reduced, the spacers can become axially displaced from their proper location. Further, current designs employ square water rod lower end plugs and square lower tie plate holes to maintain angular orientation of the water rod tab relative to the spacer structure. Such end plugs and holes are costly as compared with round end plugs and holes.